Getting 29Mh/s @ 85W on Sapphire rx 580 nitro 4gb here.
85 w gpuz ? Or from the wall ?
GPU-Z or MSI, I doubt you can have less than 100W with AMD GPU.
Eth only, yes its possible (91w at the wall @ 27Mh/s here). Could be above 27 but my elpida does not clock above 1850.
I set voltage to 831 @ 1090mhz in PBE. Lower it would hang while dual-mining.
I have seen cards running bellow 800mv, so those consume even less than 90w at the wall.
edit: dual mining @ 1130/1850 831mv = 118w at the wall